Add a set of `g:pymode_lint_*_symbol` options.

[Inspired by
Syntastic](https://github.com/scrooloose/syntastic/blob/master/plugin/syntastic/signs.vim#L10-L24),
this feature allows users to set, in their `~/.vimrc` file, a custom set of error symbols.
Merge pull request #295 from kdeldycke/develop

Add a set of g:pymode_lint_*_symbol options.